The present invention relates to a center drive support for supporting a crankshaft during machining, the center drive support comprising a rotatably drivable support member with a carrier, the carrier having at least two clamping elements with a clamping end connected thereto, the clamping elements displaceable between a clamping position and a release position for the crank shaft the clamping elements clamping the crankshaft between their clamping ends and having connected thereto a drive unit for applying a required clamping force in the clamping position during rotation of the support member.
It is known to support crankshafts during the crank pin machining within the area between the two collets by a center drive support in order to increase the machining precision (German patent 31 01 924). During cutting of eccentric workpieces, especially of crank pins at crankshafts, only those crankpins positioned within the rotational center of the center drive support can be machined. If it is desired to machine all crank pins of a crankshaft with one clamping operation, the crankshaft must be rotated about the axis of the main bearings such that all crank pins are sequentially positioned at the rotational center of the center drive support in order to be machined. The center drive support prevents rattling during the machining of the crankshaft and low quality surfaces caused thereby, respectively, prevents low machining precision. In the known center drive support the clamping elements are radially displaceably supported at an annular carrier. For adjusting the clamping elements a hydraulic drive is provided at each clamping element. The individual hydraulic drives are connected to one another via lines and are commonly supplied with a hydraulic medium. For supplying the hydraulic drive with the hydraulic medium the lines open into an annular line of the carrier. The carrier itself is rotatably driven by a worm gear when the crankshaft clamped between the clamping elements is rotated during machining. The crankshaft is machined by circumferential cutting in which the cutting takes place at the feed velocity. The crankshaft is therefore rotated at a low speed. For a crankshaft diameter of, for example, 100 mm and a feed speed of 1 m/min the center drive support is rotated at 3.18 revolutions per minute. For such a low number of revolutions the transfer of the hydraulic medium from the stationary portion of the center drive support to the rotating portion for a diameter of approximately 500 mm is still controllable with respect to the sealing elements. The sealing elements in this case rotate at a speed of 5 m/min.
There are also rotating center drive supports known which have clamping systems operating by mechanically self-restraining transfer elements, for example, threaded spindles or wedge systems. By coupling and uncoupling the clamping drive during the standstill phase of the crankshaft the clamping systems are opened or closed. During the rotation of the center drive support no clamping control via the clamping drive is possible because the drive is uncoupled during the rotation of the center drive support.
From European publication 0 264 476 center drive supports are known in which the clamping force is not generated by a self-restraining clamping system, but by spring forces. The center drive support is provided with two pivotable two-armed levers which are pivoted into their respective clamping position by a spring. These clamping elements are arranged at the rotating center drive support such that the centrifugal force increases the supporting or clamping force. With these known center drive supports it is also not possible to control the clamping force via the clamping drive during the rotation of the center drive support because the clamping elements are only subjected to a spring force.
It is therefore an object of the present invention to provide a center drive support of the aforementioned kind with which at high rotational speeds of the workpiece to be machined the clamping force exerted by the clamping elements can be controlled and maintained in a constructively simple manner.